Technical Field
The present invention relates to a sintered alloy and relates to a production method therefor. The sintered alloy may be suitably used for, for example, turbo components of turbochargers, and specifically, heat-resistant bearings that must have heat resistance, corrosion resistance, and wear resistance, and the like.
Background Art
In general, in a turbocharger fixed to an internal combustion engine, a turbine is rotatably supported by a turbine housing that is connected to an exhaust manifold of the internal combustion engine. Exhaust gas flowing in the turbine housing flows from the outer circumference of the turbine into the turbine and is discharged in the axial direction, thereby rotating the turbine. Then, a compressor, which is provided at the same shaft as the shaft of the turbine and is at a side opposite to the turbine, is rotated, whereby air to be supplied to the internal combustion engine is compressed. In such a turbocharger, in order to obtain stable supercharging pressure and to prevent the turbocharger body and the engine from being damaged, when exhaust gas flows from the exhaust manifold into the turbine housing, the amount of the exhaust gas flowing into the turbine is adjusted by separating some of the exhaust gas by switching a nozzle vane or a valve.
A bearing that receives the valve may be exposed to exhaust gas at high temperatures, and therefore, it must be superior in heat resistance and wear resistance. Moreover, since a part of the bearing may be exposed to the outside air together with the turbine housing and thereby be exposed to corrosive conditions due to salt damage or the like, the bearing must have superior corrosion resistance.
On the other hand, since a turbo component of a turbocharger may contact exhaust gas that is corrosive gas at high temperatures, it must have heat resistance in addition to corrosion resistance. Moreover, since the turbo component slidingly contacts the nozzle vane or a valve shaft, it must also have wear resistance. Therefore, for example, high chromium cast steels, wear resistant materials, and the like, are conventionally used. The wear resistant materials may be obtained by performing a chromium surface treatment on SCH22-type materials, as specified by the JIS (Japanese Industrial Standards), in order to improve corrosion resistance. In addition, as a wear resistant component that is superior in heat resistance, corrosion resistance, and wear resistance, and that is low in price, a wear resistant sintered component, which includes carbides that are dispersed in a matrix of a ferrite stainless steel, has been suggested (for example, refer to Japanese Patent No. 3784003).
Meanwhile, since transportation machines such as automobiles, which are equipped with turbochargers, are used under a wide range of conditions from warm-weather regions to cold-weather regions, the turbo component of the turbocharger is also required to be superior in wear resistance and corrosion resistance under a wide range of conditions. For example, in cold-weather regions, a salt such as NaCl (sodium chloride), CaCl (calcium chloride), etc. is sprayed on road surfaces as an antifreeze agent or a snow-melting agent. The salt melts snow and ice, whereby a large amount of water, in which the salt is dissolved at a high concentration, is present on the road surface on which the salt is sprayed. Therefore, when a transport machine travels on such a road surface, the water, in which the salt is dissolved at a high concentration, splashes on the bottom of the transportation machine body. The chloride ions contained in the water in large amounts break a passive film that formed on the surface of stainless steel, causing progressive corrosion. Accordingly, corrosion may occur in a heat resistant bearing for a turbocharger due to salt damage.
The corrosion mechanism of the salt damage is thought to occur as follows. That is, the passive film (Cr2O3) that is formed on a surface of a stainless steel reacts with Na of NaCl and H2O and forms water-soluble Na2CrO4, thereby melting away. Then, as the passive film melts, Cr is correspondingly supplied from an inside of the stainless steel, whereby the amount of Cr in the stainless steel becomes insufficient.
Such corrosion may progress even in a sintered alloy as disclosed in Japanese Patent No. 3784003 under corrosive conditions that may cause salt damage. Accordingly, a new sintered alloy having wear resistance and corrosion resistance is desired as a substitute for the above sintered alloy.